1. Field of the Invention
The present invention relates to a backlight module for a liquid crystal display device and, more particularly, to a backlight module having a frame with an impact damping design used in a liquid crystal display device.
2. Description of Related Art
With reference to FIGS. 1 and 2, FIG. 1 is a top view of the backlight module of a prior art, and FIG. 2 is an enlarged sectional view taken along the line A—A in FIG. 1. The liquid crystal display device commonly needs an additional backlight module acting as a light source. In practice, the backlight module uses a light guide plate 11 to transform the light rays provided by a lamp 12 into a uniform planar light source, and a frame 10 to accommodate and fix all the components of the backlight module.
When estimating the reliability of a backlight module, the Shock Test is one of the important tests. Two particular requirements to pass the Shock Test are that the structure of the backlight module cannot be deformed, and the components of the backlight module, such as the light guide plate 11 or the lamp 12 cannot be damaged, the lamp 12 especially. The type of the lamp 12 most used currently in the industry is the cold cathode fluorescent lamp (CCFL), which contains mercury vapor. If the cold cathode fluorescent lamp is damaged, then the user and the environment will be seriously contaminated. Therefore, under the demands of environmental protection and safety consideration, the conditions for carrying out the Shock Test have become more and more exhaustive. In particular, the light guide plate 11 often impacts the lamp 12 due to inertia when external shock energy is applied. Therefore, there must be a suitable design for fixing the light guide plate 11.
However, the design for fixing the light guide plate is confronted by strict challenges, which are illustrated as the following:
Firstly, as the information products have a tendency toward being lightweight and small in size, there is less and less space on the frame 10 for constructing a fixing structure. In addition to that, the frame 10 must reserve some space for disposing the cables of the driving ICs because more and more Driving ICs are needed. Consequently, the usable space of the frame 10 is even more reduced.
Secondly, the light guide plate 11 has to provide a uniform planar light source. The light-guiding efficiency of the light guide plate 11 and the progress paths of the light rays have been precisely simulated and experimented with. Accordingly, any irregular protrusion or indentation on the light guide plate will cause local light dispersion to be not uniform.
Thirdly, the light entry side 13 of the light guide plate 11 easily impacts on the lamp 12 when the Shock Test is carried out. Also, the light entry side 13 is a receiving side that receives the light rays coming from the lamp 12. Under the consideration of the displaying brightness, there cannot be any blocking structure located between the light guide plate 11 and the lamp 12 to prevent the light guide plate 11 from impacting on the lamp 12.
Japanese Patent No. JA-2003-66420 disclosed a fixing design of a light guide plate, which is relevant to the above-mentioned problems. With reference to FIG. 3, there is a lamp housing 22 for locating a lamp (not shown in this figure), and a frame 21 with an indentation part 24. The light guide plate has a protrusion (not shown in this figure) that corresponds to the indentation part 24 so that the light guide plate can be fixed. Near the indentation part 24, there is a slit 23 for forming a damping structure 25. However, such a design will face the problems described below:
(I) Too much space is occupied: Said patent utilizes the slit to form a damping structure resembling a cantilever beam, which occupies too much space. Especially, as the information products tend to be light, thin, and small, and more and more driving ICs are required to get higher resolution, there must be space reserved on the frame for disposing the cables. Consequently, difficulties are experienced in applying that design. (II) The root of the cantilever beam is easily broken: Because the root 26 of the cantilever is frail, it will be broken easily when the condition of the Shock Test becomes particularly severe. (III) The test and verification of the fixing design are difficult: Because the result of the Shock Test is unpredictable, the fixing design has to undergo continuous modification and practical verification from design on paper to mass production. However, the mold for producing the frame structure disclosed by that patent is hard to be modified and thus a lot of verification time is needed. (IV) The structure of the mold is complex: No matter whether it is the verification process before making the mold, or the modification process after making the mold, both the structures concerned are complex and both the processes are time-consuming, and thus the exploitation progress is seriously delayed.
TW Patent No. 525,791 disclosed another similar design for fixing the light guide plate of the backlight module. With reference to FIG. 4, there is an indentation part 34 formed on a frame 31, and a protrusion 33 located on a light guide plate 32. The protrusion 33 can engage with the indentation part 34 to fix the light guide plate 32. In order to pass the Shock Test and prevent the corner 35 of the protrusion 33 from cracking, the corner 35 is commonly designed as a circular angle that can avoid stress concentration. However, based on the consideration of usable space on the frame 31, the width W of the protrusion 33 is already very small (about 5 mm). If the corner is designed as a circular angle, the straight part 36 of the protrusion 33 will be even smaller. Combined with the manufacturing tolerance, the protrusion 33 of the light guide plate will easily come off the indentation part 34, and then the light guide plate will impact on the lamp. Hence, the shock durability of that design is still not high enough.
Therefore, it is desirable to provide a backlight module with an impact damping design to mitigate and/or obviate the aforementioned problems.